Help- parallel axis theorem

Could someone please help me with a particular question to this problem?

Consider a cube of mass m with edges of length a. The moment of inertia I_0 of the cube about an axis through its center of mass and perpendicular to one of its faces is given by I_0 = 1/6ma^2. Find I_edge, the moment of inertia about an axis p through one of the edges of the cube.
Express I_edge in terms of m and a. Use fractions rather than decimal numbers in your answer.

You know how the parallel axis theorem is I_edge = I_0 +md^2, what exactly is d in this particular problem. I tried a/2, and I tried using pythagoras' theorem to determine one of the diagonals, getting an answer of d= sqrt(2a^2)/2. Both answers where wrong when i substituted it into the equation.